when Scrolling images the image is shaking

[perlasivakrishna]

While swiping the images image is shaking little bit, and goes up finally.But if i use arrows to swipe the images its fine

• (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
• (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView if i comment these two methods, there is no shaking. is there any problem now , and what is the use of these methods.